This was due to an error in my code. I was setting content type to “application/json” for all requests instead of just the new batch ones.
For POST requests to non-batch endpoints in the V0 API that use body parameters, it seems like the content type should always be “application/x-www-form-urlencoded”
So, it appears that it was that mismatch that was causing the 401 Auth Error, as the OAuth signature wasn’t being generated correctly for non-batch api endpoints.